Various standard package types have emerged for housing microprocessors, multichip modules, memories, transistor networks, and other integrated circuits. These package types include pin grid array (PGA) packages, which include a housing with an array of conductive contact pins that extend away from the bottom surface of the package.
Sockets are commonly used to removably mount PGA packages to printed circuit boards (e.g., mother boards) or other substrates. The socket is electrically and mechanically connected to the circuit board, and the PGA package is inserted into the socket.
FIG. 1 illustrates a top view of a socket 100 in accordance with the prior art. Socket 100 includes a rigid housing 102 having a top surface, which defines a package mounting surface. An array of openings 104 in the top surface corresponds to the array of pins in the package. In addition, the array of openings 104 provides access to a corresponding array of contacts in an interior of the housing.
FIG. 2 illustrates a cross-sectional, side view of the socket 100 of FIG. 1 along section line 2—2. An array of contacts 202 resides in cavities below the top surface 204 of the housing 102. The housing captures, supports, and electrically insulates the contacts 202 from each other.
Each of the contacts 202 includes a metal body 206, which is embedded within the socket. In addition, in one embodiment, each contact 202 has a metallic depending lead 210, which extends in a perpendicular direction from the bottom surface 208 and is insertable in a through-hole of a circuit board substrate.
The metal body 206 is configured to allow insertion of a pin of a PGA package into the opening in which the metal body 206 is positioned or into a cavity in the metal body 206 itself. When the pins of a PGA package are inserted into the socket, the PGA package pins physically and electrically contact the metal bodies 206, enabling signals, power, and ground to be exchanged between a circuit board and the PGA package.
The development of microprocessor technology has caused miniaturization and high speed to become important factors in socket design. With miniaturization, the distance between adjacent contacts 202 is becoming smaller and smaller. Because of the close proximity of contacts 202 to each other, crosstalk has become an important performance issue. Crosstalk results from the coupling of the electromagnetic field surrounding an active conductor into an adjacent conductor. When too much crosstalk is present, the integrity of the signals being carried on contacts 202 decreases.
High speed performance requirements have made control of the socket impedance a significant design consideration, as well. Matched impedance at a socket is critical to minimizing signal reflections. False triggering or missed triggering of devices can occur due to reflections that are caused by impedance mismatches.
One method of reducing crosstalk and controlling impedance is to dedicate many contacts 202 as ground contacts, where these ground contacts are located adjacent to the signal carrying contacts 202. Those ground contacts provide nearby termination for the electric fields and thus reduce the coupling between the signal carrying contacts 202. By having ground contacts around the signal contacts, the characteristic impedance of the signal contacts are in tighter control, resulting in better matching between the characteristic impedances of the package and mother board. Therefore, in many high speed PGA packages and socket designs, a substantial number of contacts 202 are dedicated to ground.
The number of ground contacts necessary to ensure the required signal integrity is often expressed in terms of the signal/ground ratio. As this ratio decreases, the performance increases, but the number of pins in the socket that are able to satisfy input/output (I/O) requirements decreases. In many cases, the signal/ground ratio is nearly 1:1. Besides consuming many of the contacts that could otherwise be used for signals, ground contacts are unable to completely control the impedance or factor out the crosstalk.
As circuit frequencies continue to escalate, with their associated high frequency transients, crosstalk and impedance control increasingly become problems in socket designs. Accordingly, what is needed is a socket that has improved grounding, resulting in lower crosstalk and better controlled characteristic impedance. In addition, there is a need for a socket that is able to have a higher ratio of signal to ground pins, without sacrificing performance.